1. Field of the Invention
This invention relates to an angle encoder that serves to indicate angular positions of and the number of revolutions performed by an encoder disk and comprises incremental and absolutely coded measuring scales and associated reading means and evaluating means.
2. Description of the Prior Art
Angle encoders are used to detect angular positions of rotating or pivotally moving marts and to measure oath lengths; in the litter case the movement, e.g., the linear movement, of a sensor along a path is converted by a transducer to a rotary movement for driving the shaft of the angle encoder.
In addition to angle encoders of the kind described first hereinbefore, angle encoders are known in which the encoder disk is provided only with an absolutely coded scale.
If the angle encoders of the kind described first hereinbefore have encoder disks that have diameters which can be used in practice, e.g., of 30 or 50 mm, they can be used for a relatively high resolution of angles in dependence on the signals generated in response to the scanning of the incremental scale. To define the zero position of the encoder disk provided with the incremental scale, that encoder disk is provided with a reference mark, which must be scanned in addition to the measuring scale and the scanning of which results in the generation of a reference pulse, which synchronizes a counter, which is controlled by digital pulses derived from the signals generated in response to the scanning of the incremental scale. In response to a power failure or to a shutdown that counter is reset to zero or to a predetermined count by such reference pulse. To detect the number of revolutions performed by the encoder disk, such known encoders comprise two or more additional encoder disks, which are provided with an absolute scale, and associated reading means, which are connected by speed-reducing transmissions to each other and to the main encoder disk that is provided with the incremental scale. In the previous practice the absolute code consists of a multidigit code, which is printed on a corresponding number of circular tracks on the disk and may consist, e.g., of a gray code, and the reader extends radially of the disk and comprises scanning elements equal in number to the tracks. The resolution which can be achieved with such absolutely coded encoder disks is only a fractional part of the resolution which can be achieved by an incremental measuring apparatus. For this reason it is necessary for definite measurements to use transmissions having a relatively low speed reduction ratio, e.g., of 16:1, so that in case of disks which are 30 mm in diameter it is necessary for a detection of 1024 revolutions to provide in addition to the incrementally coded encoder disk three absolutely coded disks, the first and second of which are interconnected and connected to the main disk by transmissions having a speed reduction ratio of 16:1 and the third disk is connected to the second absolutely coded disk by a transmission having a speed reduction ratio of 4:1. The speed-reducing transmissions must be substantially free of backlash. Besides, in case of a shutdown or a power failure it will be necessary in the least favorable case to impart to the incrementally coded encoder disk almost an entire revolution until the reference pulse occurs before the absolute position of the encoder disk is again definitely defined. For this reason such angle encoders cannot be used in practice, e.g., for a direct coupling to robot arms because such angular movement of the part that is coupled to the angle encoder and consists, e.g., of a gripper moving in the interior of a hollow member would result in damage to the hollow member and to the robot. For this reason the known angle encoders used for such applications comprise an absolutely coded encoder disk so that only a much lower resolution can be achieved. The number of revolutions which are performed is again determined by absolutely coded disks, which are connected by speed-reducing transmissions.
CH-A-669,457 discloses a combined incremental and absolute measuring system for length measurements, in which the measuring scale is read by a multiline diode array, by which the illuminated scale is scanned by means of a magnifying optical system having an exactly defined magnification factor. The pixels of the array are provided with different charges in dependence on the distribution of bright and dark fields of the scale, which is provided with a bar code. In addition to the production of incremental countable signals it will be possible to effect an absolute measurement if bright and dark fields of the scale are interchanged so that the scale is additionally coded in a quasi-random form and individual scale sections are defined by different code words derived from the code. But in practice that length-measuring system has an insufficient resolution in the absolute range and difficulties are involved in an exact association of the absolute measurement with the incremental measurement. For the absolute measurement it is necessary to compare the charges of adjacent pixels of the array and to distinguish between the signals obtained from the absolute code and the signals required for an incremental measurement. Because that optical system is employed, even the slightest change of the distance from the reading unit to the measuring scale and slightest errors regarding the pitch of the scale will result in errors in the measurement. It must also be taken into account that an evaluation by which position signals are derived from the signals obtained from the absolute code will take a much longer time than the generation and evaluation of simple countable pulses generated in response to the incremental scanning of the measuring scale.